Turbo Lag vs Boost Threshold

Turbo Lag vs Boost Threshold

Last Updated February 7, 2020 | C.J. Tragakis

A turbocharger creates more power by giving the engine additional air to burn. Turbochargers differ from superchargers in that they’re powered by exhaust gas that would otherwise be vented to the atmosphere. Because of this, they will inherently have at least a little bit of turbo lag. But what is turbo lag? How is it different than a car’s boost threshold? Many drivers mistake boost threshold for turbo lag. While both of them affect what happens after you push your gas pedal down, they are both very different phenomena.

What Is Turbo Lag?

No matter what RPM level you’re at, there will be a momentary delay between when you step on the accelerator and when the turbocharger responds. This delay is what’s known as turbo lag.

A natural part of the turbocharging process involves getting your turbine to spin quickly enough to create boost (that is, additional air going into your engine’s cylinders). Unfortunately, your turbocharger won’t always be spinning quickly enough to create boost as you’re cruising along. It will only spool up when the throttle is opened, i.e. when the gas pedal is pressed. That’s what leads to turbo lag. This process must also be repeated each time you hit the accelerator.

When you let off the throttle completely, you’ll experience turbo lag once again when you press the pedal and the process repeats. However, at higher engine loads/RPM levels, the lag time will be less than at low RPMs.

Turbo Lag Process Example

What is Boost Threshold?

Unlike turbo lag, boost threshold only applies to lower RPM levels, not the entire rev range. At the lowest RPM levels, there won’t be enough pressure generated by exhaust gases to spin the turbocharger and create boost. But once the boost threshold is reached, enough gases have been generated to get the turbo spooled up and pushing more air into the engine. This is the point where you feel the boost “kick in.”

Boost Threshold Process Example

Turbo Lag vs Boost Threshold Example

While turbo lag and the boost threshold limit can feel similar when you’re trying to accelerate, the two principles are very different. Let’s take the Ford Focus ST for example.

Turbo Lag Example

Say that you’re merging onto a highway, accelerating from 40 mph to 65 mph. With the car in 4th gear, you put your foot down to begin speeding up. There will be a slight lag period before the engine truly delivers its power. It’s hardly noticeable in most modern engines, especially one with a turbocharger as small as the ST’s. But during this turbo lag period when the turbocharger is beginning to spin-up, your engine will essentially behave as if it’s naturally-aspirated. In short, you won’t have the full amount of power.

It’s hard to say how long turbo lag generally lasts; it varies heavily from car to car. But on the Focus ST, let’s say it’s around the neighborhood of 0.75 seconds (based on my internal, gut-based readout). You hardly notice it.

After the turbocharger begins to spool, it delivers even more power as the spin rate increases. Maximum power delivery is reached when you hit the boost limit of your car (or, when you ease off the accelerator).

Boost Threshold Example

If you’re driving at about 5 mph in second gear, your tachometer might have you right around 1,000 RPM. If you punch the accelerator, you won’t get boost until you reach the boost threshold RPM level. At that point, the turbo will spin enough to push air into the engine.

For the Focus ST, the boost threshold is around 1,500 RPM for most gears. However, for first gear it’s around 3,000 RPM and in second gear it’s in the neighborhood of 2,000 RPM. This isn’t the result of a mechanical function, but rather, an electronic decision controlled by the car’s computer.

Ford most likely limits boost in the lower gears to avoid/mitigate wheel spin. If you want to “unlock” some more of this power down in the lower gears, an aftermarket electronic tune will let you easily customize the way you want power delivered.

The low boost threshold that the Focus ST has in most gears means that it’s rarely a factor in highway passing (although it can be somewhat annoying when accelerating from a stop). In terms of turbo lag, the small size of the turbo means that there’s very little of it. Engineering has improved over the years, and most modern turbo cars have very minimal amounts of turbo lag.

Will I Notice Turbo Lag in My Car?

On the whole, turbo lag is noticeable in few modern cars unless you specifically look for it. The average person driving a turbo four-cylinder as a daily driver will never notice it on their commute. On the track, however, enthusiasts driving tuned vehicles with large turbochargers are sure to feel its effects.

Turbo Lag vs Boost Threshold

In a turbocharged engine, both boost threshold and turbo lag will affect how the motor behaves under acceleration. Check out this article to find out the differences between these phenomena.